Embodiments of the invention relate generally to electric drive systems including hybrid and electric vehicles and to stationary drives that are subject to transient or pulsed loads and, more particularly, to transferring energy between an electrical storage device or electrical energy source of the vehicle or drive to a load.
Hybrid electric vehicles may combine an internal combustion engine or a fuel cell and an electric motor powered by an energy storage device, such as a traction battery, to propel the vehicle. Such a combination may increase overall fuel efficiency by enabling the combustion engine and the electric motor to each operate in respective ranges of increased efficiency. Electric motors, for example, may be efficient at accelerating from a standing start, while combustion engines may be efficient during sustained periods of constant engine operation, such as in highway driving. Having an electric motor to boost initial acceleration allows combustion engines in hybrid vehicles to be smaller and more fuel efficient.
Purely electric vehicles use stored electrical energy to power an electric motor, which propels the vehicle and may also operate auxiliary drives. Purely electric vehicles may use one or more sources of stored electrical energy. For example, a first source of stored electrical energy may be used to provide longer-lasting energy while a second source of stored electrical energy may be used to provide higher-power energy for, for example, acceleration.
Energy may be transferred from the energy source of a vehicle or drive to a DC link coupled to a DC load. Often, a bi-directional boost converter may be included to decouple the energy source from a direct connection with the DC link and to boost voltage from the energy source to a higher level voltage for supply to the DC link. A high voltage boost ratio between the low side of the bi-directional boost converter that receives the voltage of the energy source and the high side of the boost converter that supplies the boosted voltage to the DC link tends to decrease system efficiency.
Increasing the system efficiency by lowering the voltage boost ratio may include having an energy source capable of supplying a voltage at or above a value near the voltage desired on the DC link. However, an uncontrollable current event may occur when the voltage of the energy source is above the voltage desired on the DC link.
It would therefore be desirable to provide a system to facilitate the transfer of electrical energy from an electrical energy source to the DC link in a controlled manner when the voltage of the electrical energy source is higher than the actual or desired DC link voltage.